Your program of walking 15 minutes at regular pace, 10 minutes at an increased pace, back to regular pace for 15 minutes, 10 minutes at increased pace, 5 minutes at regular pace, and 5 minutes at cool down pace, is the best interval walking suggestion I have seen anywhere.
Others suggest a 1-2 minute or 2-4 minute intervals, which are taxing and hard to keep track of. Your suggestions make sense, especially for folks over 50.
I am now 71, and I thrive on your walking program. It works!